The invention relates to an optical detection apparatus for chemical analyses of small volumes of samples.
Measuring apparatuses for the qualitative and quantitative chemical analyses of samples are known to the specialist in large number. For the analysis in particular of small volumes of samples, techniques are used today which are based for example on electrophoresis or chromatography, or in which the sample is examined optically without prior separation. In both cases, detection of the analytes to be identified is very often effected by means of optical methods, so that the development of new optical detection apparatus plays a very important role in respect of the instrumental improvement in the field of analysis. Even within the bounds of so-called immuno-assay, detection of biologically active substances is frequently effected by means of optical methods. Optical detection apparatuses include inter alia systems for absorption and fluorescence measurements.
In general, depending on type, detection apparatuses comprise a source of light to emit an induction light, a measuring cell for the sample, a photoelectric sensing element to receive the light coming from the sample in the measuring cell, and light-conducting means, on the one hand in order to conduct the induction light from the light source to the measuring cell, and on the other hand to conduct the light coming from the sample in the measuring cell back to the photoelectric sensing element.
In the case of fluorescence measurements, a known process is for example to bring the sample into contact with a sensor layer, e.g. a membrane applied to a substrate, the membrane containing chemical or biochemical recognition elements. As (bio-) chemical recognition elements, affinity partners of the analyte to be identified are immobilised in or on the membrane, and bind the analytes to them, for example by means of an antigen-antibody reaction. The sensor layer, which has selective sensitivity for the analyte to be identified in the sample, for example through the recognition elements, is brought to fluorescence by the induction light. When the sample makes contact with the sensor layer, the interaction between the analyte and the sensor layer or the recognition elements causes a change in the fluorescence light, for example in respect of its intensity, which is recorded by the photoelectric sensing element and is registered as the measured variable.
Frequently in the development of new detection apparatus, the main aim is to be able to carry out the analyses faster and faster and to miniaturize the apparatus more and more, so that the amount of sample employed is as small as possible. At the same time, however, the instrumental sensitivity of measurement should at the very least be maintained. In addition, it is frequently necessary to examine a sample qualitatively and quantitatively as quickly as possible for various analytes. For example, the routine examination of blood in hospitals and laboratories requires that the sample, namely blood, is examined in respect of different analytes, e.g. the partial pressure of so-called blood gases, such as carbon dioxide and oxygen, the concentration of electrolytes, such as H.sup.+ (pH value), Na.sup.+, K.sup.+, Ca.sup.2+, Cl.sup.-, or for its metabolite content, such as lactate, glucose or creatinine. In agro-technology also, it is often necessary to example a sample in respect of several analytes.
An optical detection apparatus, which is equipped for the examination of small volumes of samples, is disclosed in EP-A-0 616 211. The fundamental idea here is to keep to a minimum the number of optical transitions--by which is meant the transitions in the optical path between areas of differing optical density--between the source of light and the photoelectric sensing element. This is achieved, whereby the induction light emitted from the light source is conducted by a special photoconductor, which has a refractive index gradient, for example a gradient index (GRIN) lens, to a capillary tube containing the substance, and the light coming from the sample in the capillary tube is conducted by a further photoconductor to the photoelectric sensing element. In all the transition areas between the individual optical elements, there is an index adaptation medium, the refractive index of which conforms basically with that of the wall material of the capillary tube. In addition, this index adaptation medium provides a mechanically stable link of the individual optical elements. This concept, called "pigtailing", has the advantage that the induction light and the light from the sample, with the exception of the interior of the capillary tube, always travel in the media at an essentially constant optical density. Thus, disadvantageous scattered light effects are reduced. The mechanically stable set-up has the advantage that vibrations do not disturb the optical path of light.
Although such detection apparatuses have proved to be very advantageous, there are a few applications for which they are not optimally suited. For example, if the detection apparatus is to be designed for examination in respect of several analytes in one sample, in general several light sources would be needed, as well as several induction light conductors and several photoconductors for the sample, which respectively include GRIN lenses, optical light-wave conductors or similar optical elements, in order to effect several essentially separate light paths for the individual measurements. For optimum functioning, it is necessary that the distinct elements, such as light source, photoconductors, optical filters, photoelectric sensing elements, are adjusted extremely precisely relative to one another. Each of the elements must be individually very carefully positioned with respect to its adjacent elements, and subsequently fixed by joining with the index adaptation medium. Thus, the preparation of the optical section is already associated with a relatively high amount of work. In addition, the induction light conductor and the sample photoconductor must be of very exact dimensions and positioned and secured exactly on the capillary tube in order to attain illumination of the sample which is as efficient as possible, and in order to optimise the intensity of light from the sample contacting the photoelectric sensing element. Also, the GRIN lenses frequently used as photoconductors are relatively expensive compared with other optical elements, and some experience and time is needed to incorporate these in the optical light path with exact dimensioning and positioning. Normally, these operations must be carried out by hand. Even if such an expenditure of time and money is not of primary importance to applications for research purposes, for off-site applications outside of research laboratories in respect of efficient mass-production of these detection apparatuses, it is much more relevant. As the number of different analytes to be identified increases, the production outlay also increases, since for each of the individual light paths, the elements forming it have to be adjusted individually.
As the number of analytes to be measured increases, however, the size of the detection apparatus also increases, if only for the reason that more optical elements have to be present. Since for example GRIN lenses must have certain dimensions for physical reasons, the apparatus restricts the limits of miniaturization.